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Tuesday, February 19, 2008

Mountains of CO2

Carbon sequestration looks like an essential tool for bridging the global energy economy between its fossil-fuel present and its greenhouse-gas-free future. Despite the recent cancellation or reorientation--depending on your perspective--of the Department of Energy's Futuregen initiative, the deployment of carbon capture and sequestration (CCS) technology to coal-fired power plants is quite possibly the only thing that will keep them viable, at least in the developed world. But carbon sequestration looks expensive, both in terms of the cost per ton of the carbon dioxide it keeps out of the atmosphere, and in the energy it would consume in the process. A discovery at UCLA could change that calculation and make CCS cheaper and more effective than with current technology.

Understanding why CCS is so difficult today requires a little knowledge about combustion and gas separation chemistry. When fossil fuels are burned in air, the resulting flue gas still contains all of the nitrogen of the original air, plus water vapor and CO2 from the hydrogen and carbon in the fuel, along with pollutants resulting from fuel impurities such as sulfur or the conversion of some of the nitrogen into nitrous or nitric oxide. It's not easy to separate the diluted CO2 from the rest of these gases, and the current industrial mechanisms for doing this require a lot of hardware and use a lot of energy.

The main current alternative involves burning the fuel in pure oxygen, either partially in a gasifier or completely in O2-blown combustion. The end-result of both of these processes is mainly water and CO2, which are easily separated, allowing the latter to be compressed and injected into depleted oil reservoirs or other geological storage. Unfortunately, producing the quantities of oxygen required by this approach adds significant costs and reduces net power output. In order for CCS to become cheap and easy, we need a simple, low-cost way to extract CO2 from a gas stream, and that's just what the researchers at the California NanoSystems Institute at UCLA appear to have developed, in the form of novel zeolite crystals with a very high affinity for CO2.

Zeolites are already used in a variety of industrial processes. One of their main features is their incredible porosity, which creates almost unimaginable surface area in a very small volume of material. Maximizing that surface area is important, because of the way other chemicals react with the catalysts or linking structures deposited on these surfaces. A pound of the new CO2-absorbing zeolite would have an effective surface area greater than 200 acres. And according to the paper the developers published in Sciencelast week, a liter of zeolite could soak up 83 liters of CO2 gas, storing it until deliberately released. That means that power plant flue gas could be routed through beds of zeolite and emerge virtually CO2-free. From there, the CO2 could be released in pure form for compression and geological storage, or, if the zeolite proves cheap enough to make, it could simply be carted off for disposal. That strategy might even work for soaking up the CO2 from car engines, before it gets into the air.

Before we get too excited about the prospect of burying all of our CO2 in the form of zeolites, however, we need to realize how much we're talking about. The US produces almost 6 billion tons per year of CO2 from the combustion of fossil fuels. At 83 liters of CO2 stored per liter of UCLA's zeolite, we're still talking about 9 cubic miles of material, every year. And in terms of your car's exhaust, it would take about 175 gallons of the stuff to soak up the CO2 produced from burning 12 gallons of gasoline. In other words, while it could be used as a sort of "catalytic converter" for automotive CO2, your car would need a zeolite tank ten times larger than its fuel tank, and you'd have to empty it at every fill-up.

Even if the material in question might not be the best long-term disposal method for CO2, this could still be a hugely important development. If this zeolite proves to be as easy to mass-produce as those already in wide use, and if the CO2 it absorbs can easily be stripped out later, it could make carbon capture and sequestration extremely cost-competitive, compared to other ways of reducing greenhouse gas emissions. And because we already have an industry producing zeolites for other purposes, we might be able to mass produce this substance soon enough to make a difference, depending on how long it takes to get out of the laboratory.